This invention relates to a torque split control apparatus for use with an automotive vehicle supported on a pair of primary drive wheels and a pair of secondary drive wheels.
For example, Japanese Patent Kokai No. 2-270640 discloses a torque split control apparatus for use with a 4WD type automotive vehicle. The conventional torque split control apparatus is arranged to transmit the drive from the engine to the primary drive wheels of the vehicle and to the secondary drive wheels of the vehicle through a torque distributing clutch capable of varying a torque transmit ted to the secondary drive wheels. The torques transmit ted through the torque distributing clutch include a control torque T.sub.1, a starting torque T.sub.2 and an initial torque T.sub.3. The control torque T.sub.1, which is directly proportional to the difference between the front and rear road wheel rotational speeds, provides a desired turning characteristic to the vehicle. When the vehicle starts, the vehicle condition dependent torques T.sub.2 and T.sub.3 are calculated. The starting torque T.sub.2, which is in direct proportion to the throttle valve position, provides a sufficient traction without no rear road wheel spin when the vehicle starts on a road surface having a low friction factor. The initial torque T.sub.3, which is in direct proportion to the vehicle speed, is used to ensure the control torque T.sub.1 to be transmitted without the influence of the oil pressure response delay at a low temperature (for example, -25.degree.).
With the conventional torque split control apparatus, however, the vehicle condition dependent torques T.sub.2 and T.sub.3 are always transmitted through the torque distributing clutch regardless of the road surface friction factor and the ambient temperature. This results in fuel economy loss peculiar to 4WD vehicles.